摘要:

Previous paleomagnetic observations for the carbonates of the Lower Ordovician Knox Group have indicated that ancient magnetizations in these rocks are of the same age as the late Paleozoic Alleghenian Orogeny. Rock magnetic properties strongly suggest magnetite as the carrier of the magnetization, but the textural and crystalline characteristics, sizes, morphologies, and mineral associations of these magnetites are poorly known. We have examined magnetic extracts and iron oxides in thin sections with scanning (SEM) and scanning/transmission (STEM) electron microscope techniques to determine whether the observed iron‐oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers. Several different forms of magnetite in limestones and dolomites, which in places are host to Mississippi‐Valley type deposits, are documented by imaging and energy‐dispersive analysis using SEM and STEM, by X ray diffraction and electron diffraction patterns using STEM. The magnetite is either spherical with a dimpled surface or nonspherical in the form of void‐filling single grains or grain aggregates. Most of the iron oxides have the composition of pure end‐member magnetite, but occasional titanomagnetite and hematite, including rare zincian hematite, have been observed (only in limestone). Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium‐feldspar, illite, and quartz. Some iron oxides have, in fact, inclusions of K‐feldspar and quartz. Some of the magnetite (spherical and nonspherical) is polycrystalline; this implies that the larger observed grains may consist of single domains or pseudo‐single domains. This provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single‐domain (but interacting?) subgrains, on the basis of remanent coercivities and blocking temperatures. We interpret the pure end‐member magnetite to be authigenic, having formed at approximately the same time as the K‐feldspars, which in nearby areas have yielded late Paleozoic radioisotopic ages (278–322 Ma). The Knox carbonates therefore are inferred to carry a chemical rémanent magnetization. Iron‐rich clays or original iron‐titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution‐precipitation processes. These processes require rock‐fluid interactions which are thought to be related to migrating connate brine

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12327

年代:1990

数据来源: WILEY

摘要:

Piston cores from the Ontong‐Java Plateau collected during the Eurydice expedition have been used to estimate regional geomagnetic paleointensity. Rock magnetic measurements of the sediments show that (1) the remanence is carried by stable pseudo‐single domain magnetite, (2) the grain sizes and concentrations are relatively uniform with respect to the normalization technique and (3) susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field. We therefore have measured the rémanent intensity (both natural and demagnetized to 15 mT) and susceptibility of archive quarters of three cores at centimeter intervals using an automated through‐bore magnetometer. A further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores. One of our cores displays high coherence over a broad range of frequencies, while for two others, coherence is insignificantly different from zero at the 95% level of confidence. Oxygen isotopic records, paleontological datum levels and the Brunhes‐Matuyama boundary provide an independent temporal framework for inter‐core comparison. When the two cores which pass our acceptance criteria are placed on a common time scale, the coherence between the normalized intensity records is significantly different from zero at well above the 95% level of confidence, suggesting some common input signal over the same band of frequencies. Furthermore, there is a great deal of similarity between the records from the Ontong‐Java Plateau and a record from core RC10‐167 taken some 3000 km to the north. The distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows (0–5 Ma), with a slightly smaller standard deviation. The inter‐core agreement within a single region, the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field. Spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years, over the period 0–350 ka; this feature is however lacking for t

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12337

年代:1990

数据来源: WILEY

摘要:

Magnetic properties (natural remanent magnetization (NRM), susceptibility (χ), Curie point temperature (Tc), saturation isothermal remanent magnetization (IRMs), and Köenigsberger ratio (Q)) and opaque mineralogy were determined for basalts, diabases, gabbros, peridotites, and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites (15°N, 17°N, 23°N (the Snake Pit hydrothermal field), 26°N (the TAG hydrothermal field) on the Mid‐Atlantic Ridge, and 42°N (the Sea Cliff hydrothermal field) on the northern Gorda Ridge). The magnetomineralogy is interpreted in terms of deuteric, hydrothermal, metasomatic, and ambient seawater alteration. Evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases, gabbros, and ultramafic rocks. This lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low‐temperature alteration in young surface basalts to more pervasive high–temperature alteration in diabases, gabbros, and peridotites. The studies reveal distinct correlations between magnetization intensity, thermomagnetic behavior, and magnetic mineralogy, grain size, style, and intensity of alteration and rock type. Basalts have the highest NRM intensities and the lowest Curie points (105°–294°C); basalts with NRM/IRMsratios less than 10−2may have been remagnetized and have NRM intensities andQvalues much lower (one–third and one–half, respectively) than basalts with NRM/IRMsratios of 10−2or greater. In the oceanic lithospheric suite examined, remanent (Q>1) magnetization is present in basalts and induced magnetization (Q<1) occurs in metabasalts and the other rock types. Layer 2A basalts are the source of median valley magnetic anomalies. The magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2B dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites. This shift is influenced by fluctuations in the Curie isotherm related to the duration of active magma chambers and convective hydrothermal cells. The magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere, magmatic cooling history, and

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12351

年代:1990

数据来源: WILEY

摘要:

The last exposure to sunlight of detrital feldspars and quartz grains in Quaternary sediments can be dated directly by thermoluminescence (TL) methods, but the effectiveness of zeroing of the light‐sensitive TL signal by daylight varies widely, depending upon details of the sedimentation processes. To document this variability and to help develop objective criteria useful for future sample selection and laboratory procedures, I have collected and analyzed several eolian, fluvial, marine deltaic, and lacustrine samples from less‐than‐100‐year‐old deposits as well as suspensions from sediment plumes. Also, underwater spectral irradiance was measured during recovery of suspensions. TL properties of polyrnineralic (feldspar dominated) 2–4 μm and 4–11 μm size fractions were measured in the laboratory. Unexpectedly, some light‐sensitive TL was found to remain in all samples. However, the light‐sensitive TL was more effectively zeroed in some samples (a proximal loess, a sandy braided‐channel deposit, tidal flat and upper‐delta‐front muds, and thin clayey lacustrine laminae) than in others (a silty braided‐channel deposit, submarine fan muds, and silt‐bearing lacustrine laminae). For the first group the minimum relict paleodose obtained with the partial‐bleach technique is about 0.4±0.3 Gy, corresponding to a relict TL age estimate of about 200 years for these sediments. Thus similar sediments (excluding fluvial deposits) as young as about 2 ka can be accurately and routinely dated without significant correction for a zero‐point error if one uses the partial‐bleach technique coupled with integrated optical‐bleaching energy fluences of less than ≃ 1 J cm−2and wavelengths only above about 550 nm. Also, some simple optical‐bleaching tests are reported that have a potential use in sample selection and in provenance studies of some surficial detrital sediments. Finally, a simple mathematical model is presented that helps account for the gross featur

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12375

年代:1990

数据来源: WILEY

摘要:

Records from four New Zealand tide gauges were analyzed in order to determine the trends in mean sea level which have occurred in New Zealand since the beginning of this century. Although a substantial difference in the computed sea level trends between tide gauges was considered a possibility, due to the location of the gauges in relation to the boundary of the Australian and Pacific tectonic plates, the analyses did not show this to be the case. They indicated rising trends in sea level of 1.3, 1.7, 2.3, and 1.4 mm/yr for Auckland, Wellington, Lyttelton, and Dunedin, respectively, and together gave a mean trend of 1.7 mm/yr for the east coast of New Zealand as a whole. These figures not only add to the global picture of sea level change but also provide initial evidence suggesting little or no recent differential vertical motion across this portion of the Australian and Pacific tectonic plates. In addition, they provide conclusive evidence that the orthometric height datums established in New Zealand from tide gauge data collected in the early part of this century are in need of substantial revision.

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12399

年代:1990

数据来源: WILEY

摘要:

Conventional methods for measuring ice bulk density do not allow one to detect three‐dimensional changes in density in a nondestructive manner. An X ray computed tomography (CT scanner) was used to determine ice core density. This scanner has led to remarkable achievements in the field of medical science to obtain a nondestructive cross‐sectional representation of the human body. In this paper the relation between density and the CT value ordinarily obtained by the CT scanner is discussed for the ice/air mixtures similar to ice core samples. The influence of impurities on the density determination is also considered. Images, taken at intervals of 2 mm, detected the internal air bubble distribution of an ice core. Three‐dimensional density distribution was also evaluated on the volume elements with dimensions of 1.1 by 1.1 by 2 mm. Results of CT scanner measurements were compared with those of the conventional hydrostatic method. Comparisons made on compressed ice samples with various densities showed close agreement. The results show that the CT scanner can be used to nondestructively determine the three‐dimensional distribution of ice core density and that the CT scanner is an extremely valuable tool for making detailed measurements of spatial variation in density of ice cores as well as for examining the internal st

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12407

年代:1990

数据来源: WILEY

摘要:

This paper continues to explore the possibility of identifying the times of increased probability (TIPs) of occurrence of a strong earthquake by premonitory intermediate‐term seismic activation in lower‐magnitude range, as defined in the algorithm M8. A TIP refers to a 5‐year period and an area whose linear size is several times larger than that of an incipient earthquake. In the course of the global test of the algorithm we applied it retrospectively to the data from the National Earthquake Information Center worldwide earthquake catalog (since the regional Japan Meteorological Agency catalog was unavailable to authors), related to the segment of circum‐Pacific belt between latitudes 20°N and 48°N, from the south of Kuril arc to the Taiwan and Izu‐Bonin arc. The worldwide catalog allows us to diagnose TIPs for earthquakes above magnitude 7.5 from 1975 on. Five out of six earthquakes withM≥ 7.5 occurred within the TIPs diagnosed by the algorithm M8 with a priori fixed parameters. The missed earthquake occurred at the final stage of formation of the corresponding TIP. Altogether, the TIPs occupy less than 20%, and times of expectation occupy about 8%, of the total space‐time domain considered. There are also current TIPs covering about half of the territory. Application of the additional algorithm of localization, named “Mendocino Scenario,” indicates that the southern part of the Japan trench is an area deserving particular attention. The results obtained here by no means constitute the definitive earthquake prediction but merely outline the territories for more detailed and comprehensive analysis. If confirmed by regional data, the results may be used for implementation of some safety measures and evaluation of other precursors, includ

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12413

年代:1990

数据来源: WILEY

摘要:

The estimation ofQvalues and/or source corner frequenciesfcfrom single‐station narrow‐band recordings of microearthquake spectra is a strongly nonunique problem. This is due to the fact that the spectra can be equally well fitted with low‐Q/high‐fcor a high‐Q/low‐fcspectral models. Here, a method is proposed to constrain this ambiguity by inverting a set of microearthquake spectra for a three‐dimensionalQmodel structure and model source parameters seismic moment (Mo) and corner frequency (fc) simultaneously. The inversion of whole pathQcan be stated as a linear problem in the attenuation operatort* and solved using a tomographic reconstruction of the three‐dimensionalQstructure. ThisQstructure is then used as a “geometrical constraint” for a nonlinear Marquardt‐Levenberg inversion ofMoandfcand a newQvalue. The first step of the method consists of interactively fitting the observed microearthquake spectra by spectral models consisting of a source spectrum with an assumed high‐frequency decay, a single‐layer resonance filter to account for local site effects, and additional “whole path attenuation” along the ray path. From the obtainedQvalues, a three‐dimensionalQmodel is calculated using a tomographic reconstruction technique (SIRT). The individualQvalues along each ray path are then used asQstarting values for a nonlinear iterative Marquardt‐Levenberg inversion ofMoandfcand a “new”Qvalue. Subsequently, the “new”Qvalues are used to reconstruct the nextQmodel which again provides starting values for the “next” nonlinear inversion ofMo,fc, andQ. This process is repeated until the “goodness of fit measure” indicates no further improvement of the results. The method has been tested on a set of approximately 2800Pwave spectra (0.9

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12423

年代:1990

数据来源: WILEY

摘要:

A new method to simultaneously invert forQstructure and source parameters was used on a set of 635 microearthquakes (0.9

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12439

年代:1990

数据来源: WILEY

摘要:

We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990]that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–

ISSN:0148-0227    

DOI:10.1029/JB095iB08p12449

年代:1990

数据来源: WILEY